Laser radiation is used in numerous techniques for treatment of the human eye. In some of these techniques focused laser radiation is utilised for the purpose of ablating (resecting) eye tissue. In this case it is necessary to direct the beam focus onto the eye in controlled manner, so that the ablation takes place at the desired position on the eye. But by virtue of movements of the eye during the treatment the eye may change in its position in relation to the treatment laser beam. This may then result in a discrepancy between a specified position and an actual position of the ablation.
For this reason it is desirable to track the movements of the eye and to take them into account in the control of the beam focus. For the purpose of acquiring the eye movements, use is made of an eye-tracker. At present, a two-dimensional eye tracking is generally conventional, which is based on the acquisition of the pupillary margin of the eye by only one camera. From the light/dark jump in contrast at the pupillary margin (iris), the pupillary centre is calculated which then serves as orientation coordinate for the laser ablation. Control of the treatment laser radiation is then effected by taking into account the position of the pupillary centre ascertained by the eye-tracker. However, the position of the pupillary centre does not always lie on the axis of symmetry of the eye or on the optical visual axis of the patient (for example, by virtue of asymmetrical displacement of the pupillary centre in the case of varying pupillary size, or deviation from circular symmetry in many patients). Such a deviation may result in suboptimal treatment outcomes.
In order to avoid such inaccuracies in the laser treatment resulting from the shift of position of the pupillary centre, the tracking of the pupil can be supplemented by a tracking of the limbus, which is oriented with respect to the invariable light-dark transition of the white sclera (sclerotic coat of the eye) to the iris. Prominent displacements of the pupillary centre can consequently be detected and taken into account in the ablation program as a so-called pupil-centre-shift correction (PCSC).
Overall, for the purpose of tracking the eye movement (eye tracking) the state of the art has hitherto utilised two-dimensional camera-image acquisition. Positional defaults derived therefrom may, however, be faulty, since the actual eye movements take place in three-dimensional space and consequently three translational movements as well as three rotational movements have to be described. Furthermore, eye-trackers that are based on camera-based two-dimensional image-recordings enable only the indirect acquisition of three-dimensional data by computationally intensive reconstruction. Meanwhile, camera-based eye-trackers have become available that enable a five-dimensional or six-dimensional eye tracking. In this connection, by an additional projection onto the eye of a pattern of light consisting of fringes and by the acquisition of these fringes (registration of the curvature, position and deformation of the fringes), a locational position and orientational position of the eye are inferred. But, here too, the registration process is intensive in terms of computation and time. Therefore the image-rate of eye-trackers used hitherto is limited in its speed and is often too slow for a correction of position in the course of the treatment of the eye with laser light. In addition, the camera systems utilised for this purpose merely detect light that has been scattered or reflected by the eye of the patient, which is why it is necessary to ensure appropriate illumination of the eye (which, however, may also have a disturbing effect on the treatment) and at the same time to avoid incidence of light from other secondary illuminations from the room onto the eye.